I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for optimizing a cellular network using empirical data received by base stations.
II. Description of the Related Art
Currently, cellular systems are designed using simulation design tools such as those described in U.S. patent application Ser. No. 08/515,772, entitled “INTERACTIVE GRAPHICAL TOOLS FOR CELLULAR SYSTEM DESIGN”, filed Aug. 16, 1995, and in U.S. patent application Ser. No. 08/535,999, entitled “CDMA NETWORK PERFORMANCE ANALYSIS TOOL”, filed Sep. 29, 1995, both of which are assigned to the assignee of the present invention and are incorporated by reference herein. The data provided by these simulations is limited by the amount of information regarding the terrain and propagation path characteristics that are provided to the simulation.
The aforementioned patent application Ser. No. 08/535,999 describes in detail a method and apparatus for planning a code division multiple access wireless telecommunications network. An electronic representation of wireless telecommunications system can be configured within a given market area and the operation of that wireless telecommunications system simulated. The simulation is performed using a set of databases that contain terrain and population information associated with the market area over which the wireless network in configured. To perform the simulation a composite propagation loss matrix and a demand and service vector are generated using the terrain and population information. Once the propagation loss matrix and the demand and service vector are generated an analysis of the reverse link is performed. Subsequently, an analysis of the forward link is performed. During both the reverse and forward link analysis the multiple iterations of analysis are performed until a stable result is achieved. Upon completion of the reverse and forward link analysis, the results of the simulation are displayed in a graphical manner for examination.
The aforementioned patent application Ser. No. 08/515,772 describes in detail a method and apparatus for providing network planning information in an interactive manner. A user selects a planning tool and inputs a selection point. If a line of sight cursor tool has been selected, a set of points on a display are set indicating a line of sight exists from those points to the selected point. If a path loss cursor tool has been selected a set of points on a display are set indicating that path loss to the selection point is less than a certain threshold. Alternatively, a set of point on the display are set to various colors or shades indicating a range of path loss amounts to the selected point. In the preferred embodiment the invention incorporates the use of a computer system.
A second step that is frequently used to determine network parameters is to perform “dry runs” on the equipment by sending test mobile stations into the field. However, these tests are generally very time consuming and limited to generally a small set of predefined locations. On the other hand, during commercial operation users which access the network freely from any location within the coverage provide important and useful information to the network operator via the PSMM's.
The use of code division multiple access (CDMA) modulation techniques is but one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and are incorporated by reference. The method for providing CDMA mobile communications was standardized by the Telecommunications Industry Association in TIA/EIA/IS-95-A entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (hereafter IS-95).
In the just mentioned patents, a multiple access technique is disclosed in which a large number of mobile telephone users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as cell base stations or cell-sites) using code division multiple access (CDMA) spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA techniques results in much higher spectral efficiency than can be achieved using other multiple access techniques.
A method for simultaneously demodulating data that has traveled along different propagation paths from one base station and for simultaneously demodulating data redundantly provided from more than one base station is disclosed in U.S. Pat. No. 5,109,390 (the '390 patent), entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR COMMUNICATION SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein. In the '390 patent, the separately demodulated signals are combined to provide an estimate of the transmitted data which has higher reliability than the data demodulated by any one path or from any one base station.
Handoffs can generally be divided into two categories—hard handoffs and soft handoffs. In a hard handoff, when a mobile station leaves and origination cell and enters a destination cell, the mobile station breaks its communication link with the origination cell and thereafter establishes a new communication link with the destination cell. In soft handoff, the mobile station completes a communication link with the destination cell prior to breaking its communication link with the origination cell. Thus, in soft handoff, the mobile station is redundantly in communication with both the origination cell and the destination cell for some period of time.
Soft handoffs are far less likely to drop calls than hard handoffs. In addition, when a mobile station travels near a cell boundary, it may make repeated handoff requests in response to small changes in the environment. This problem, referred to as ping-ponging, is also greatly lessened by soft handoff. The process for performing soft handoff is described in detail in U.S. Pat. No. 5,101,501, entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM” assigned to the assignee of the present invention and incorporated by reference herein.
An improved soft handoff technique is disclosed in U.S. Pat. No. 5,267,261 (the '261 patent), entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM”, which is assigned to the assignee of the present invention and incorporated by reference herein. In the system of the '261 patent, the soft handoff process is improved by measuring the strength of “pilot” signals transmitted by each base station within the system at the mobile station. These pilot strength measurements are of assistance in the soft handoff process by facilitating identification of base station handoff candidates capable of providing reliable communication with the mobile station.
The base station candidates can be divided into four sets. The first set, referred to as the Active Set, comprises base stations which are currently in communication with the mobile station. The second set, referred to as the Candidate Set, comprises base stations which have been determined to be of sufficient strength to provide reliable communications with the mobile station. Base stations are added to the candidate set when their measured pilot energy exceeds a predetermined threshold TADD. The third set, referred to as the Neighbor Set, is the set of base stations which are in the vicinity of the mobile station (and which are not included in the Active Set or the Candidate Set). And the fourth set is the Remaining Set which consists of all other base stations.
When a CDMA base station is configured, there are many parameters that can potentially vary. For example, each IS-95 base station transmits a neighbor list that identifies the base stations which are most likely to be able to communicate with a mobile station in the area. The members of the neighbor list may simply include the base stations that are most closely located to that base station and are frequently determined in accordance with simulation programs like those described above. However, it is possible that the cells that are able to provide reliable communications with the base station are not simply the base stations most closely located. Some base stations that are close to that base station may be blocked by obstructions. Conversely, some distant base stations may be able to provide communications to a mobile station within the coverage of the base station owing to clear propagation path. In addition, configuration of the base stations, including the tilt of the antenna, the azimuth angle of the sector antennas, the total transmit power of the base station and the physical location of the base station are all capable of changing to further optimize the network configuration.
In an IS-95 communication system, the mobile station sends a Pilot Strength Measurement Message when it finds a pilot of sufficient strength that is not associated with any the of the Forward Traffic Channels currently being demodulated or when the strength of a pilot that is associated with one of the Forward Traffic Channels being demodulated drops below a threshold for a predetermined period of time. The mobile station sends a Pilot Strength Measurement Message following the detection of a change in the strength of a pilot under the following three conditions:                1. The strength of a Neighbor Set or Remaining Set pilot is found above the threshold TADD.        2. The strength of a Candidate Set pilot exceeds the strength of an Active Set pilot by more that a threshold (TCOMP).        3. The strength of a pilot in the Active Set of Candidate Set has fallen below a threshold (TDROP) for greater than a predetermined time period.The Pilot Strength Measurement Message identifies the base station and the measured pilot energy in decibels.        
In current systems, the Pilot Strength Measurement Messages (PSMMs) are used to perform handoff operations and are then discarded by the system. However, there is valuable information contained in these messages that can be used to optimize a cellular network. Therefore, a need exists for a method and apparatus that uses the information contained in the transmitted PSMMs to assist in the optimization of network configuration in a cellular communication system. And to provide a method for configuring a cellular network based on true empirical data, as opposed to the limited accuracy of simulated data and data compiled in dry runs.